Several experiments in our laboratory have shown that P300 ERP amplitude in rats is a decremental function of conditioned stimulus proximity to primary reinforcement in behavioral chains. We have also demonstrated robust P300 ERPs to stimuli predicting the occurrence as well as the omission of expected reinforcers. These findings support the hypothesis that the P300 is a correlate of the brain’s response to recognizing a conditioned reinforcer. In all the experiments we have conducted, the P300 was measured well after the response was established. The present experiment investigated the growth of the P300 during the shaping of the response to food delivery magazine (click training) and subsequent lever pressing. During this training a target tone (5.5 KHz, 70 db SPL) predicted the click of the pellet delivery magazine while a random non-target tone (2.5 KHz, 70 db SPL) was presented on a 8:1 non-target to target ratio. Seven rats received this training across 12 training sessions that consisted of 60 target tone-click-food pairings and 480 non-target tones. The mean amplitude of the P300 ERP to the target tone increased significantly across the 12 days reaching 128 µV and 95 µV on Days 11 and 12 respectively. Latency of the peak amplitude was about 800 msec and did not change significantly across the training sessions. Since the P30 ERP amplitude increased gradually across the sessions, during the time that the target stimulus was acquiring conditioned reinforcing properties, these findings further support the working hypothesis that the P300 ERP is a correlate of the brains recognition of a conditioned reinforcing stimulus.